prompt: add nix segment

scripts/prompt/src/main.zig

@@ -51,6 +51,7 @@ pub fn main() !void {
                     return error.MissingEnv,
                 10,
             ),
+            .nix_name = @import("nix.zig").findNixShellName(),
         };
 
         var buf: [1024 * 8]u8 = undefined;

scripts/prompt/src/nix.zig

@@ -0,0 +1,22 @@
+const std = @import("std");
+
+/// If we're likely in a Nix shell, return the name of that shell or "?" if it's unknown,
+/// null otherwise.
+pub fn findNixShellName() ?[]const u8 {
+    return if (isInNixShell()) std.posix.getenv("name") orelse "?" else null;
+}
+
+fn isInNixShell() bool {
+    if (std.posix.getenv("IN_NIX_SHELL")) |_| return true;
+
+    var path_iter = std.mem.splitScalar(
+        u8,
+        std.posix.getenv("PATH") orelse return false,
+        ':',
+    );
+
+    while (path_iter.next()) |p|
+        if (std.mem.startsWith(u8, p, "/nix/store")) return true;
+
+    return false;
+}

scripts/prompt/src/prompt.zig

@@ -22,6 +22,7 @@ const symbols = struct {
     const root = "";
     const watch = "";
     const jobs = "";
+    const nix = "󱄅";
 };
 
 pub const Options = struct {
@@ -29,6 +30,7 @@ pub const Options = struct {
     mode: FishMode,
     duration: u32,
     jobs: u32,
+    nix_name: ?[]const u8,
 };
 
 pub fn render(writer: anytype, options: Options) !void {
@@ -56,6 +58,7 @@ fn Renderer(comptime Writer: type) type {
             try self.setStyle(.{ .background = left_color });
             try self.renderDuration();
             try self.renderJobs();
+            try self.renderNix();
             try self.renderCwd();
             self.renderGit() catch |err| {
                 switch (err) {
@@ -147,6 +150,15 @@ fn Renderer(comptime Writer: type) type {
             try self.writer.print(" {s} {}", .{ symbols.jobs, self.options.jobs });
         }
 
+        fn renderNix(self: *Self) !void {
+            if (self.options.nix_name) |name| {
+                try self.drawLeftSep(.Blue);
+                try self.setStyle(.{ .background = .Blue, .foreground = .Black});
+
+                try self.writer.print(" {s} {s}", .{symbols.nix, name });
+            }
+        }
+
         fn renderCwd(self: *Self) !void {
             var cwd_buf: [512]u8 = undefined;
             const cwd = try std.posix.getcwd(&cwd_buf);